Companies increasingly involve consumers in the process of developing advertising and other marketing actions. An important question that has not been explored is whether brands benefit from communicating to consumers who had not been involved in the co-creation process that a target ad was developed by a fellow consumer. The authors propose a skepticism–identification model of ad creator influence, which hypothesizes that disclosing to an audience that an ad was created by a consumer triggers two opposing effects: skepticism about the competence of the ad creator and identification with the ad creator. Four studies demonstrate that the effectiveness of disclosing advertising co-creation depends on factors that hinder skepticism and heighten identification with the ad creator. Specifically, attributing the ad to a consumer is shown to increase persuasion when the audience (1) has limited cognitive resources to scrutinize the message, (2) is given background information about the ad creator that enhances source similarity, and (3) has high loyalty toward the brand. The implications of these findings on marketing theory and practice are discussed.